Recent advancements on the fabrication of organic micro- and nanostructures have permitted the strong collective light–matter coupling regime to be reached with molecular materials. Pioneering works in this direction have shown the effects of this regime in the excited state reactivity of molecular systems and at the same time have opened up the question of whether it is possible to introduce any modifications in the electronic ground energy landscape which could affect chemical thermodynamics and/or kinetics. In this work, we use a model system of many molecules coupled to a surface-plasmon field to gain insight on the key parameters which govern the modifications of the ground-state potential energy surface. Our findings confirm that the energetic changes per molecule are determined by effects that are essentially on the order of single-molecule light–matter couplings, in contrast with those of the electronically excited states, for which energetic corrections are of a collective nature. Hence the prospects of ultrastrong coupling to change ground-state chemical reactions for the parameters studied in this model are limited. Still, we reveal some intriguing quantum-coherent effects associated with pathways of concerted reactions, where two or more molecules undergo reactions simultaneously and which can be of relevance in low-barrier reactions. Finally, we also explore modifications to nonadiabatic dynamics and conclude that, for our particular model, the presence of a large number of dark states yields negligible effects. Our study reveals new possibilities as well as limitations for the emerging field of polariton chemistry.